As someone who's only good football skills is good shots with both feet, here's my approach once I decided to improve my weak foot.
Go to a football field, go to the penalty spot and start shooting with your left foot. This will be awkward and you will hit like a 4 year old. That's fine, you will get the feel for it after a while.
Once you're comfortable doing the motion with your left foot, start aiming in the corners of the net and call your shots. Rinse and repeat for the other techniques. The reason you have a weak foot is not because it is weak, it is lack of technique. Once you start to practice using it, you will get better at it.
There's nothing more to it, practice makes perfect.